1. Field of the Invention
This invention relates to size reduction of particles of explosives. More particularly, the invention pertains to an improved method for rapidly and safely grinding explosive materials to a small particle size with reduced variation in particle size distribution.
2. State of the Art
Size reduction of explosive materials has historically been accomplished by either (a) dissolution and recrystallization under carefully controlled conditions, or (b) grinding of the dry explosive. Grinding equipment such as fluid energy mills or jet mills are typically used. These grinders have no moving parts in contact with the material undergoing size reduction. The particles are ground by fluid jets which cause the particles to travel in a "racetrack" course, so that the particle size is reduced by interparticle collision. Alternatively, ball mills or pin mills are used. Nevertheless, the use of fluid-energy mills and other dry grinding methods for explosives is considered to be inherently hazardous. Excessive energy input into a single particle may result in catastrophic detonation. Such high explosives as cyclotrimethylenetrinitramine (RDX) and cyclotetramethylenetetranitramine (HMX) are considered to be too dangerous to be used in the pure form in ammunition. Various desensitizing agents are combined with the explosive materials which enable their use in ammunition and useful detonable products.
Even with insensitive materials such as coal, steam is used as a carrier in production scale fluid-energy mills to minimize the risk of spontaneous combustion and possible explosion.
Wet grinding, i.e. grinding of a slurry of the solid material in an "inert" liquid such as water, is considered to be much less dangerous, because the liquid lubricates the solids and readily absorbs energy.
U.S. Pat. No. 3,239,502 of Lee et al. describes one method currently used for preparing cyclotetramethylenetetranitramine (HMX) of small particle size, i.e. less than 325 mesh. Crude HMX is diluted with a non-solvent liquid such as water, methanol, ethanol or the like. The resultant slurry is recirculated by passage through a piping system including a pump or pumps, and throttling valves or orifices. The recirculating treatment is conducted for a period of at least 10 (ten) hours to gently grind the HMX particles. A cyclone separator is used to separate the desired fines from the larger particles. The latter are returned to the grinding circuit for additional size reduction. The grinding period is very long, typically about 16 hours, and there is considerable batch to batch variation in mean particle size as well as in the distribution of particle size.
More recently, ultrasonic energy has been proposed for size reduction of coal. For example, U.S. Pat. No. 4,156,593 Tarpley Jr. describes the size reduction of coal particles for separating contaminants such as pyrite and clay therefrom. Coal is slurried in an aqueous liquid containing a leaching agent and a penetrant/embrittling agent. Fragmentation in the presence of ultrasonic cavitation is facilitated by the natural porosity of coal.
U.S. Pat. No. 4,410,423 of Walsh describes the use of ultrasonic energy to enhance the acid dissolution of sodium fluoride and cryolite. Alkaline ore containing the sodium fluoride, cryolite, and insoluble alumina is reduced in particle size by dissolution of the fluoride and cryolite. However, the released alumina particles and carbon particles in the slurry are not reduced in size by the ultrasonic treatment.
Generation of an ultrasonic field in liquids may result in cavitation capable of producing high local pressures, i.e. several tens of thousands of atmospheres. It is believed that the gas bubbles which are created have high internal temperatures as well, e.g. 5000 to 10,000 degrees C. Microscopic flames are known to occur in the liquid, and the ultrasonic treatment has been shown to ionize water, degrade organic compounds, melt metals, and erode solid surfaces. Sonification is also known to significantly increase the detonation rates of explosives.
The high explosives RDX and HMX are known to be particularly sensitive to impact, with impact sensitivities of 0.45 and 0.52 kgm, respectively. When the conditions are such that a gas may be adiabatically compressed, with a rapid increase in temperature, and subsequently collapsed, the sensitivity of the explosive material is known to be enhanced. Such conditions are known to exist, and are in fact desirably created, in liquids subjected to ultrasonic generation.